1. Field of the Invention
The present invention relates to subsurface safety valves for controlling fluid flow in tubing or conduit disposed in a wellbore penetrating subsurface strata.
2. Background of the Related Art
A subsurface safety valve, also known simply as a safety valve, is an apparatus that is used in various wellbore types (e.g., subsea, platform, land-based) to provide a “fail-safe” mechanism for closing the wellbore to prevent the uncontrolled release of hydrocarbons or other downhole fluids. A safety valve is typically actuated in emergency situations, such as blowouts, to provide a pressure barrier (oftentimes in cooperation with blowout preventers) and safeguard local personnel, equipment, and the environment.
U.S. Pat. No. 4,161,219 discloses a safety valve that employs a flapper valve that is spring-biased towards a position closing a fluid passageway in the safety valve body, and a flow tube that is normally positioned so as to yield the biasing spring of the flapper valve and secure the flapper valve into a position opening the fluid passageway. The flow tube is also spring biased towards an upper position that releases the flapper valve, but the flow tube is normally urged towards a lower position in which the flapper valve is secured by the application of a control fluid pressure from the surface. In the event of an emergency, such as a blowout, the control fluid pressure is reduced to permit the spring bias of the flow tube to urge the flow tube towards its upper position, thereby releasing the flapper valve so that its biasing spring urges the flapper valve towards the position closing the fluid passageway.
When the flapper valve of a conventional safety valve is released from its opening position, fluid pressure in the fluid passageway as well as the flapper valve's spring bias apply a closing force to the flapper valve. In high-flowrate wellbores, this closing force effects a relatively rapid closing motion that causes the flapper valve to impart substantial loading conditions on the lower end of the flow tube, as well as the flapper hinge mechanism. In other words, the flow tube is not moved fast enough by its biasing spring to avoid the closing movement of the flapper mechanism. As a result of the expected loading forces between the flapper mechanism and flow tube, the flow tube must typically be strengthened by way of increased wall thickness and the flapper hinge mechanism strengthened by increasing material strengths and/or material web sections to avoid incapacitating damage. The increased wall thickness of the flow tube and/or increased hinge web section effectively compromises the safety valve's internal diameter and therefore reduces the resultant fluid flow capacity through the safety valve.
A need therefore exists for a safety valve that mitigates the risk of damage by the extremely high loading forces between a flapper mechanism and a flow tube.
A further need exists for a safety valve that mitigates such risk without compromising fluid flow capacity.